Observing the truth: Recording Earth’s changing flow of energy

Abstract

The receipt of energy within our atmosphere is the fundamental driver of the climate system. Our ability to accurately measure this exchange of energy and establish a reliable understanding of its trend and variability is fundamental to comprehending the current state of Earth’s climate. This dissertation presents three novel assessments of Earth energy budget observations: trends in the polar top-of-atmosphere solar absorption and thermal emission, constraining atmospheric energy transport from observations, and examining the covariability of energy flux components and meridional energy transport.Satellite observations reveal that decreasing surface albedo in both polar regions is increasing the absorption of solar radiation, but the disposition of this absorbed energy is fundamentally different. In the Arctic, the rates of increasing absorbed solar radiation and thermal emission are remarkably similar (0.98 and 0.94 Wm-2 dec-1 respectively), with solar absorption variability explaining two thirds of the annual thermal emission variability.Conversely, Antarctic thermal emission is not responding to the increasing (though not yet statistically significant) solar absorption of 0.59±0.64 Wm-2 dec-1 with less than a third of the annual thermal emission variability explained by accumulated solar absorption. The Arctic is undergoing rapid adjustment to increasing solar absorption resulting in no change to the net energy deficit, while increasing Antarctic solar absorption represents additional energy input into the Earth system being taken up by the Southern Ocean.To close the atmospheric energy budget, the meridional transport of energy must be recorded, representing the net energy moved by winds across a latitude circle. Here, a method for calculating energy transport from observed energy fluxes is presented, calculated as the integral (accumulation) of the net energy input into the atmosphere from one pole to the other. Observations tend to underestimate annual mean energy transport compared to reanalysis, attributed to reduced poleward gradients in the observed surface turbulent heat fluxes. There is however, close agreement in the variance with correlations of up to 0.8 and 0.5 in the midlatitudes and tropics respectively between calculations. The observed energetic framework presented here facilitates a novel assessment of the energetic fluxes consistent with energy transport variability.Variability in the meridional heat transport is associated with net energy gains andlosses within approximately ±15° either side a latitude band. Anomalous gradients of atmospheric latent heating is the most important term for describing the variability of midlatitude heat transport. Enhanced evaporation within the tropics provides an excess of energy which is then transported poleward and lost through all energy fluxes, radiation, turbulent fluxes, and the atmospheric tendency. The atmospheric tendency (the storage of energy in the atmosphere) is the most important component for heat transport into the polar regions through the convergence of energy transport which is not immediately lost from the atmosphere. These results provide a benchmark for assessing variability in the climate system with the use of observations.